1. Field of the Invention
The present invention relates to a direct injection diesel engine and in particular relates to a direct injection diesel engine wherein the cleaning of exhaust gas and improvement in fuel consumption are effected by performing homogenious charge compression ignition combustion.
2. Description of the Related Art
In recent years, demands for environmental measures relating to internal combustion engines, in particular diesel engines, have become increasingly strong and various means such as diesel particulate filters (DPF) that capture soot such as black smoke or NOx catalysts that effect cleaning by reducing NOx are being provided. In addition, various combustion systems are being studied in order to respond to demands for lowering fuel consumption with the aim of basically eliminating black smoke or NOx and, in addition, lowering CO2.
However, DPF or NOx catalysts of the continuous regeneration type, which have recently been attracting attention, are restricted in regard to the temperature at which they function effectively; in particular, they have problems such as not functioning effectively in the region of low/medium engine load, where the temperature of the engine exhaust gas is low. Also, in the case of the above DPF or catalysts, when the reducing agent HC is added from the fuel, this is accompanied by a deterioration of fuel consumption.
In view of the above, it is preferable from the technical standpoint to improve combustion itself.
One way of doing this which has recently attracted attention is homogenious charge (or pre-mixing) combustion, in which ignition is effected after pre-mixing the fuel within the combustion chamber by altering the fuel injection timing from the normal timing to make use of the time from the start of fuel injection up to ignition. It is known that low NOx and low smoke can be achieved by such homogenious charge combustion.
Known ways of achieving homogenious charge combustion include early injection of fuel relative to the fuel injection timing in the case where ordinary diffusion combustion is performed (typically, BTDC 10° to ATDC 10° ) and the achievement of homogenious charge combustion by prolonging the ignition lag period by retarding the timing.
However, retarding the fuel injection timing always results in a timing wherein the combustion period departs from the top dead center such that deterioration of thermal efficiency is difficult to avoid, and it is therefore undesirable since this is accompanied by deterioration of fuel consumption. Early fuel injection is therefore considered preferable, since the combustion period can be set in the vicinity of the top dead center so as not to be accompanied by deterioration of fuel consumption.
However, homogenious charge combustion in which fuel is injected earlier than usual (this is called homogenious charge (or pre-mixing) compression ignition combustion) is subject to the following problems.
First of all, homogenious charge compression ignition combustion cannot be performed in all operating regions of the engine. This is because higher rates of fuel injection under full load result in ignition starting before the engine reaches the vicinity of the top dead center, resulting in severe knocking, which damages the engine. Another reason is that even if ignition starts in the vicinity of the top dead center, subsequent combustion proceeds all at once, resulting in a rise in combustion temperature and increased NOx.
Next, in homogenious charge compression ignition combustion, since the fuel is injected when the piston is lower than normal, if the injection angle is not correctly set, the fuel that is injected sticks to the inside wall of the cylinder, tending to cause an increase in unburnt HC and/or oil dilution, which tends to result in a deterioration in fuel consumption.